The invention relates to a method of and to an apparatus for operating machines of the type wherein two or more rolls define one or more nips for running sheets or webs of paper, textile material, metallic or plastic foil or the like. Typical examples of such machines are calenders (including supercalenders) and so-called glazing machines. More particularly, the invention relates to improvements in methods of and in apparatus for operating machines wherein at least one nip is defined, at least in part, by a roll with bending compensation, namely a roll having a more or less rigid carrier surrounded by a cylindrical shell which is deformable to a desired extent adjacent different portions of the nip, and wherein different portions of the shell along the nip are deformable and/or otherwise adjustable relative to the carrier and relative to the adjacent roll or rolls by a plurality of discrete actuators in the form of hydrostatic bearing elements, fluid-operated cylinder and piston units which can move the ends of the carrier and/or other types of moving and/or deforming means.
In certain machines of the above outlined character, the pressure of fluid medium which is admitted into the actuators is a function of the desired or reference value profile of a load parameter in the nip. When the reference value for the actuator controlling a selected zone of the nip is changed, this entails pressure changes in actuators for the other zones of the nip. As a rule, the actuators receive pressurized fluid (such as oil) by way of conduits which contain adjustable pressure regulating valves, i.e., the pressure of fluid which is admitted to a particular actuator is changed by adjusting the setting of the respective pressure regulating valve.
In most instances, the load parameter which is of importance in machines to which the present invention pertains is the line load (namely the magnitude of the force per unit length of the nip) or the compressive strain (this is the magnitude of the force per unit area) along the nip. It is important and highly desirable to ensure that the load parameter (be it the line load, compressive strain or a variable which is a function of line load and/or compressive strain) along the nip of two rolls in a calender or a glazing or analogous machine will assume a value which matches the desired (reference) value as well as that the reference value will be matched or closely approximated when the machine is in actual use. This cannot be accomplished in presently known machines because it is not possible to measure the forces which develop in the nip of two rolls when a calender or an analogous machine is in actual use.
German Offenlegungsschrift No. 28 25 706 of Biondetti proposes to employ a simplified mechanical model of the machine and to measure the distribution of forces in the nip of two rolls in the model. To this end, the rolls which are to define the nip are replaced by beams, and pressure monitoring gauges are installed in longitudinally spaced-apart zones of the nip. Each pressure monitoring gauge is operatively connected with a pressure generating element at that side of one of the beams which faces away from the nip. Each pressure generating element corresponds to a hydrostatic or other bearing element of an operative calender or a like machine. Each zone of the nip of the two beams (imitation rolls) is controlled by a regulator having a first input for the application of a signal denoting an adjustable reference value of the load parameter in question and a second input for the actual value (denoted by a signal which is transmitted by the corresponding pressure monitoring gauge) of the load parameter in the respective zone of the nip. The regulator compares the two signals and transmits to the associated pressure generating element a signal which is indicative of the difference (if any) between the actual and monitored values of the load parameter in the corresponding zone of the nip of the two beams. Such signal from the regulator to the pressure generating element selects the appropriate pressure of fluid in the pressure generating element. The regulator is further connected with the pressure generating elements (actuators) of the machine so that the pressure in the nip of two rolls in the machine is regulated in the same way as the pressure in the nip of beams forming part of the model. If the reference value is changed for any given zone of the nip, the stiffness of the beams causes that such change of reference value in the given zone alters the conditions prevailing in the neighboring zones of the nip; this, in turn, causes the regulator or regulators for the neighboring zone or zones to bring about corresponding changes of conditions prevailing in the respective zone or zones of the nip i.e., to bring about corresponding changes of pressure of fluid in the respective pressure generating element or elements of the model and in the respective actuator or actuators of the machine whose operation is to be controlled as a function of monitoring the conditions in the nip of the beams forming part of the model.
A calender or a like machine is often a very large unit wherein the length of the rolls is in the range of several meters. Therefore, it is very difficult to build a model which is a sufficiently close replica of the original machine. In addition, important parameters of a calender or a like machine are changed, for example, when one or more rolls having elastic coats are treated (e.g., milled) with attendant reduction of their diameters. This alters the weight and the rigidity of the thus treated rolls. Alternatively or in addition thereto, the parameters of a calender or a like machine will change in response to changes of overhanging weights, for example, when it is necessary to change one or more guide rollers for the running web, strip or sheet of material which is to be treated during travel through the nip. Such changes cannot be taken into consideration in models of the type disclosed by Biondetti.
German Pat. No. 31 17 516 to Surat discloses a method according to which external corrections of the pressure regulating signal for a given group of bearing elements in a calender or an analogous machine entail the generation of auxiliary correction signals which are applied to the controls for neighboring groups of bearing elements so as to compensate for changes in those zones of a nip under the control of neighboring groups of bearing elements, namely for changes which are attributable to external corrections of the pressure regulating signal for the given group of bearing elements. This method completely disregards the conditions which prevail in the nip. Though a change in one zone of the nip entails compensatory changes in other zones of the nip, the applied auxiliary correction signals do not ensure that the conditions prevailing in the other zones will remain unchanged, i.e., that they will match or at least approximate the desired conditions.
Controlled deflection rolls with bending compensation are disclosed in numerous United States and foreign patents of the assignee. Reference may be had, for example, to U.S. Pat. Nos. 4,394,793, 4,425,489 and 4,457,057.